This invention relates to a method of drilling a well including supplying gas with the drilling liquid.
The present invention is particularly concerned with under-balanced drilling which is a simple concept in which the hydro static pressure within the well generated by the head of fluid from ground level to the point of drilling is reduced to the point where the formation pressure is higher than the hydro static pressure, allowing the well to flow while drilling. The result of under-balance drilling is a well drilled without formation damage.
While simple in concept, this may be difficult to attain in the practical situation since many reservoirs are pressure depleted and the conventional technique of simply reducing the drilling fluid density to the point where the well starts to flow will not work. In many reservoirs, the reservoir pressure is below the hydrostatic pressure of fresh water which is of course the minimum to which the hydrostatic pressure of the drilling liquid can be reduced without addition of gas.
In such circumstances where the reservoir pressure is below this level, it is necessary, if the well is to be drilled under-balance, to use an alternative technique.
In some cases it is possible to effect drilling using air or other gas as the circulating medium without the conventional drilling liquid. This has a number of disadvantages as follows:
a) The downhole motor life is significantly reduced and is less predictable since the downhole motors are generally designed to be run using the conventional drilling liquid as the power source. The significant differences between drilling liquid or mud and the air or gas have led to problems in the application of these motors to air drilling.
b) Hole cleaning is a problem at inclinations above 50.degree.. Because air is compressible, the flow rate changes with pressure. Also, because of its much lower lifting capacity, air requires annular velocities much greater than that of the conventional drilling liquid. Particularly the air volume required to clean the hole is three times greater than the recommended flow rate for the motor. Such excessive flow rates therefore often cause premature failure of the motor.
c) The air drilling system cannot be used with the conventional MWD (Measurement While Drilling) telemetry system used to communicate downhole information to the surface. Typical MWD equipment pulses the mud system by a downhole pulse generator to vary the pressure within the mud. These variations in pressure are then detected at the surface for the purposes of detecting the coded information transmitted through the mud pulse system from the downhole measuring transducers. Because air is compressible, it cannot be pulsed effectively. Therefore conventional mud pulse (MWD) does not work in an air drilled hole. Other communication systems for example using radio waves are available but are very much less effective.
In the air drilling system, air at high pressure is injected into the drill string and the majority of the air injected is released at an air by-pass sub immediately above the downhole motor. The remaining air passes through the motor, powers the motor and exits through the bit nozzles to effect cleaning.
This system of course generates a very low pressure in the hydrostatic head at the production zone allowing the well to flow during the drilling action so that production materials and the pumped air are communicated to the surface through the annular space between the well and the drill string.
In view of the above difficulties, air drilling has achieved little success. Therefore attempts have been made to reduce the hydrostatic pressure of the drilling liquid by providing nitrogen (or air) injection into the well to supplement the conventional drilling mud and thus reduce the hydrostatic pressure of the mud.
There are two techniques in use. The first technique involves the injection of nitrogen into the drill string so that the drilling liquid and nitrogen are simultaneously pumped into the drill string for communication down the drill string to the drilling tool. This technique has been found to work reasonably well, however there are concerns with motor performance due to the passage of the gas through the motor with the liquid and in addition the MWD mud pulse tools will not work since the gas carried within the liquid is compressible so that the pulses are lost in the elastic material between the downhole mud pulse generator to the transducer at the surface.
The second technique is to inject nitrogen into the annulus of the build section of the well. This requires the installation of a nitrogen injection string while running the intermediate casing and therefore requires more complexity and is not applicable for use while drilling re-entry wells.
It is well known that the MWD system is effective and widely used. The MWD system can be used during motor drilling operations to provide accurate and frequent drift angle and azimuth data. As well, frequent tool face updates are provided while motor drilling. The sensor package, as well the power unit and pulser unit are all part of the drill string.
The inability therefore to use MWD systems has very much limited the use of gas to effect under-balanced drilling and instead many operations have accepted the damage which occurs due to the high hydrostatic pressure forcing the drilling liquid into the producing zone.